A fuel cell is a device which generates electrical energy by converting chemical energy, derived from a fuel supplied to the cell, directly into electrical energy by oxidation of the fuel in the cell. A typical fuel cell includes a casing which houses an anode, a cathode and an electrolyte. Appropriate fuel material and oxidant are supplied respectively to the anodes and cathodes, the fuel and oxidant react chemically to generate a useable electric current, and the reaction end product is withdrawn from the cell. A relatively simple type of fuel cell involves use of hydrogen and oxygen as the fuel and oxidant materials, respectively. The hydrogen combines with the oxygen to form water while at the same time generating an electrical current. More specifically, hydrogen is consumed at the fuel cell anode releasing protons and electrons as shown in equation (1) below. The protons are injected into the fuel cell electrolyte. The electrons travel from the fuel cell anode to the anode terminal, through an electrical load, back to the cathode terminal, and into the cathode of the cell. At the cathode, oxygen, electrons from the load and protons from the electrolyte combine to form water as shown in equation (2) below.
Anode Reaction EQU H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- ( 1)
Cathode Reaction EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.- .fwdarw.H.sub.2 O (2)
A great advantage of a fuel cell is that it converts chemical energy directly to electrical energy without the necessity of undergoing any intermediate steps, for example, combustion of a hydrocarbon or carbonaceous fuel as takes place in a thermal power station.
Fuel cells can be classified into several types according to the electrolyte used. Modern relatively high performance fuel cells include electrolytes such as aqueous potassium hydroxide, concentrated phosphoric acid, fused alkali carbonate and stabilized zirconium oxide. The electrodes invariably include a catalyst for promoting the reactions that take place on respective electrodes in the fuel cells. Suitable catalysts include nickel, silver, platinum and, in the case of the stabilized zirconium oxide electrolyte, base metal oxides.
General Electric in the 1960's commenced work on the development of a solid polymer fuel cell (SPFC). Such a cell had a number of potential advantages. It could operate on a hydrogen containing fuel and an oxidant feed such as air or pure oxygen. In one embodiment, the SPFC could operate on reformed hydrocarbons such as methanol or natural gas as the fuel source and air as the oxidant.
Since the electrolyte in a SPFC is solid, substantial pressure differences between the fuel and the oxygen streams can be tolerated. This simplifies pressure control and, in particular, allows for higher pressures to exist in the oxidant stream. This leads to increased performance, particularly when air is used as the oxidant. An SPFC is advantageous in that it can be operated at temperatures below the boiling point of water at the operating pressure. Accordingly, water as the end product is generated in the liquid state.
More specifically, a typical SPFC uses a solid polymer ion exchange membrane as electrolyte between the anode and cathode. The solid polymer ion exchange membrane permits the transmission through the membrane of hydrogen ions, but is substantially impervious to the passage of hydrogen and oxygen molecules. The ion exchange membrane has thereon negatively charged sites chemically attached to the polymer. The ion exchange membrane is sandwiched between the anode and cathode. Typically, a platinum catalyst is added to the anode and cathode to increase the rate of reaction.
In a single cell arrangement, two fluid flow field plates (anode and cathode plates) are provided. The plates act as current collectors, provide electrode support, provide means for access of the fuel and oxidant to the anode and cathode surfaces, respectively, and provide for removal of water formed during operation of the cell.
The cell assembly is held together by tie rods and end plates. Feed manifolds are respectively provided to feed the fuel (hydrogen, reformed methanol or natural gas) to the anode and the oxidant (air or oxygen) to the cathode via the fluid flow field plates. Exhaust manifolds are provided to exhaust excess fuel and oxidant gases and water formed at the cathode. Multi-cell structures comprise two or more such sandwich combinations connected together in series or in parallel to increase the overall power output of the assembly as required. In such arrangements, the cells are typically connected in series, wherein one side of a given plate is the anode plate for one cell, and the other side of the plate is the cathode plate for the adjacent cell and so on.